1. Field of the Invention
The present invention is singularly directed to the production of oil from subsurface deposits, primarily below 1,000 feet. Unlike systems used for the recovery of less viscous fluids, water by way of example, the recovery of oil is required to be accomplished from relatively deeper deposits, using significantly smaller diameter casing.
By way of example, water pumping systems, by virtue of the use of casing diameter of 12 inches and greater, are able to make practical use of higher RPM pumps, which are, by nature, larger in diameter. Moreover, because of the relatively shallow nature of such wells, such pumps are easily driven from a source of power located at the surface. This is because the drive shaft for transmitting motive power to a high revolution pump is coincidently shorter, and the amount of bearing support required is within practical limits. Clearly, the longer the drive shaft, the more bearing support required, with a commensurate increase in construction and maintenance costs.
Yet another distinguishable difference between oil and water wells is the inevitable presence of natural gas in an oil deposit, which is not found in water deposits. Oil wells accommodate gasses by using a conduit within the casing to relieve pressure and harvest the gasses. Remembering that oil well casings are typically less in diameter, the use of agricultural and other water recovery systems which are 12 inches and more, would be extremely difficult to adapt to oil production.
Mechanical lifting of oil from subsurface deposits is a common, indeed necessary, means of producing the world's hydrocarbon energy needs. The apparatus for accomplishing this needed task falls predominantly into five strategies or categories: rod pumping, gas lift, hydraulic pumping, electric submersible pumping, and progressive cavity pumping. Each type has its strong and weak points.
Rod pumping, the most common type of artificial lifting apparatus, consists of a piston type pump located downhole where it is submersed in the deposit in the well. The technique is to actuate the pump with a reciprocating rod string extending from the downhole pump to a pumping unit at the surface. This type of system is reliable, easily serviced, and satisfactory for most wells. However, rod pumping is not particularly well suited to deep, gassy, or abrasive fluid applications, i.e., where sand, salts and like particulate is found in the deposit, and has limited rate and depth capability due to the tensional strength limitations of the rod string.
Yet another problem with such systems becomes evident if a rod string breaks, and such is not uncommon. The cost in both time and effort to fish out the pump from the bottom of the well, repair or replace the string, and return the pump to the appropriate depth, is high, yet borne regularly by those in the business, because there is no other way. The deeper the well, of course, the longer the string, and the greater the load on the string as it is reciprocated to operate the pump. Not surprisingly, the rate of failure of such strings is significantly higher.
Another fluid recovery system in wide use is referred to generally as a gas lift system and consists of injecting high pressure gas into a fluid filled tubing at depth, to lighten the fluid column, and cause the fluid to flow to the surface. Gas lift systems work well in moderate rate, moderate depth applications. It is insensitive to gassy or abrasive fluids, because the equipment is mechanically simple and inexpensive, and the systems are very reliable. Gas lift requires a source of gas, is energy inefficient, expensive to run and operate because of the compression requirements, and a poor option in low rate applications.
The currently preferred option for production of deep, low to moderate rate wells is referred to simply as hydraulic pumping. A typical system consists of a downhole piston pump which is connected to a downhole piston motor. The motor is actuated by high pressure hydraulic fluid injected down a string of tubing to the downhole pump-motor assembly. The reciprocating movement of the motor actuates the pump, which lifts the fluid in the deposit to the surface.
The tradeoff with hydraulic pumping is that hydraulic pumps are expensive to install and operate, and do not handle abrasive or gassy fluids well. They require high pressure hydraulic pumps at the surface, hydraulic fluid (usually crude oil) storage and treating facilities, and at least two strings of tubing.
Hydraulic jet pumps employ identical surface equipment and tubing requirements used in hydraulic pump systems such as described above, but replace the piston pump/motor assembly with a venturi-type jet assembly that uses Bernoulli's principle to "suck" the produced fluid into the stream of hydraulic fluid passing through the jet. The mix of hydraulic and produced fluid crude then flows up to the surface. Hydraulic jet pumps handle gassy fluids well, but are limited in the effective draw down they can generate and are energy inefficient.
A more recent approach to producing subsurface deposits has become available with the commercial exploitation of the progressive cavity pump.
Progressive cavity pumping (PCP) consists of a Moyno type pump downhole, which is actuated by a rod string that is rotated by a motor at the surface. PCPs are particularly well suited for delivering viscous, abrasive fluids. The surface and bottom hole equipment is simple and reliable, and energy efficiency is good. Progressive cavity pumps handle gas satisfactorily, but the system has depth and rate limitations and will mechanically fail if the volume of fluid entering the pump is less than what the pump can lift, and the well "pumps off".